Self-propelled pneumatic tools for making small diameter holes through soil are well known. Such tools are used to form holes for pipes or cables beneath roadways without need for digging a trench across the roadway. These tools include, as general components, a torpedo-shaped body having a tapered nose and an open rear end, an air supply hose which enters the rear of the tool and connects it to an air compressor, a piston or striker disposed for reciprocal movement within the tool, and an air distributing mechanism for causing the striker to move rapidly back and forth. The striker impacts against the front wall (anvil) of the interior of the tool body, causing the tool to move violently forward into the soil. The friction between the outside of the tool body and the surrounding soil tends to hold the tool in place as the striker moves back for another blow, resulting in incremental forward movement through the soil. Exhaust passages are provided in the tail assembly of the tool to allow spent compressed air to escape into the atmosphere.
Most impact boring tools of this type have a valveless air distributing mechanism which utilizes a stepped air inlet. See, for example, Sudnishnikov et al. U.S. Pat. No. 3,410,354, issued Nov. 12, 1968. The step of the air inlet is in sliding, sealing contact with a tubular cavity in the rear of the striker. The striker has radial passages through the tubular wall surrounding this cavity, and an outer bearing surface of enlarged diameter at the rear end of the striker. This bearing surface engages the inner surface of the tool body.
Air fed into the tool enters the cavity in the striker through the air inlet, creating a constant pressure which urges the striker forward. When the striker has moved forward sufficiently far so that the radial passages clear the front end of the step, compressed air enters the space between the striker and the body ahead of the bearing surface at the rear of the striker. Since the cross-sectional area of the front of the striker is greater than the cross-sectional area of its rear cavity, the net force exerted by the compressed air now urges the striker backwards instead of forwards. This generally happens just after the striker has imparted a blow to the anvil at the front of the tool.
As the striker moves rearwardly, the radial holes pass back over the step and isolate the front chamber of the tool from the compressed air supply. The momentum of the striker carries it rearwardly until the radial holes clear the rear end of the step. At this time the pressure in the front chamber is relieved because the air therein rushes out through the radial holes and passes through exhaust passages at the rear of the tool into the atmosphere. The pressure in the rear cavity of the striker, which defines a constant pressure chamber together with the stepped air inlet, then causes the striker to move forwardly again, and the cycle is repeated.
In some prior tools, the air inlet includes a separate air inlet pipe, which is secured to the body by a radial flange having exhaust holes therethrough, and a stepped bushing connected to the air inlet pipe by a flexible hose. See Sudnishnikov et al. U.S. Pat Nos. 3,410,354, issued Nov. 12, 1968 and 4,078,619, issued Mar. 14, 1978.
These tools have been made reversible by providing a threaded connection between the air inlet sleeve and the surrounding structure which holds the air inlet concentric with the tool body. See, for example, Sudnishnikov et al. U.S. Pat No. 3,756,328, issued Nov. 12, 1968. The threaded connection allows the operator to rotate the air supply hose and thereby displace the stepped air inlet rearwardly relative to the striker. Since the stroke of the striker is determined by the position of the step, i.e., the positions at which the radial holes are uncovered, rearward displacement of the stepped air inlet causes the striker to hit against the tail nut at the rear of the tool instead of the front anvil, driving the tool rearwardly out of the hole.
The screw reverse mechanism described in the foregoing U.S. Pat. 3,756,328 has proven inconvenient. To reverse the tool, it is often necessary to rotate the air hose as many as 12-18 times. This can prove difficult when the tool has travelled a great distance because of the length of hose that must be rotated.
The foregoing tool also employs a large, heavy tailpiece which is threadedly secured in the rear end of the tool body. In practice this type of tailpiece has proven very difficult to remove, making the tool hard to disassemble for servicing or replacement of worn parts. The '328 tool also utilizes a large, cylindrical shock absorber through which the exhaust passages are formed. This shock absorber must generally be bonded to the adjoining casing and tailpiece, again rendering the tool difficult to assemble and disassemble.
The tailpiece of the '328 tool and other conventional tools has a rearwardly tapered rear portion with a central circular hole through which the air hose extends. As shown in Bouplon U.S. Pat. No. 4,662,457, issued May 5, 1987, the hose is generally secured to the air inlet by a metal coupling. Exhaust air must pass between the metal coupling and the rim of the tailpiece in order to escape from the tool. During reverse movement, small stones can become jammed in the space between the coupling and the tailpiece, making it impossible to rotate the hose to switch modes.
The tool body of the foregoing known tools is generally made from a solid steel bar which is drilled out to form the tubular tool body. This method of fabricating the tool body is results in a large amount of wasted material, increasing substantially the cost to manufacture such a tool.
Known tools also produce a loud, high-pitched ringing sound during operation. This sound results from the undampened blows of the striker against the body, and poses a potential health hazard to the tool user.
The present invention addresses the foregoing drawbacks of known tools.